Optical transmission device and optical transmission system employing the same

ABSTRACT

An optical transmission device which reduces optical noise in an optical transmission system. The optical transmission device includes a core light amplifying unit and a first buffer light amplifying unit for amplifying a first signal light from a first transmission path and an amplified second signal light from the core light amplifying unit. The first buffer light amplifying unit supplies the core light amplifying unit with the first signal light, and supplies the first transmission path with the amplified second signal light. Also provided is a second buffer light amplifying unit for amplifying a second signal light from a second transmission path and an amplified first signal light from the core light amplifying unit. The second buffer light amplifying unit supplies the core light amplifying unit with the second signal light, and supplies the second transmission path with the amplified first signal light.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical transmission deviceand an optical transmission system. More particularly, the presentinvention relates to an optical transmission device and an opticaltransmission system suitable for low-noise transmission.

[0002] In an attempt to satisfy a requirement of lowering the cost foran optical transmission system, a wavelength divisional multiplexingoptical transmission system, which transmits different wavelengths ofsignal lights in one single optical transmission fiber, has beenconsidered. In particular, a bi-directional optical transmission system,which transmits different wavelengths of light signals in a singleoptical transmission fiber bi-directionally, is suitable when exchangeof information is needed interactively between the two connectedstations. Under such a technical background, it has become moreimportant to provide an optical amplifier applicable to a bi-directionaloptical transmission system.

[0003] Japanese Patent Laid-open No. Hei 6-85369 describes as aconventional apparatus an optical amplifier. The optical amplifierincludes apparatus for multiplexing or demultiplexing signal lights in aforward of a reverse direction toward both ends of a doped fiber. Theoptical amplifier is capable of sharing the use of one opticalamplifying medium and one optical pumping source in the forward or thereverse direction, and is applicable to a bi-directional opticaltransmission system, the constitution of which is simple.

[0004] Japanese Patent Laid-open No. Hei 9-98136 describes anotherexample of an optical amplifier which is capable of controlling theindividual wavelength output even if there occur variations in signalwavelength multiplexity.

[0005] The optical amplifiers disclosed by the above-identified Japanesepatent applications have various disadvantages in their practical use asdescribed below. It is generally known that, in a one-directionaloptical amplifier having one doped fiber, a signal light input loss at astep previous to the doped fiber is attributed to a degradation in theS/N ratio.

[0006] “Optical Amplifiers and Their Application” (Ohm Publishing, May,1992, pp 5-3[1]), describes that it is essential to combine an opticalisolator at the front of doped fiber for suppressing reflexed amplifiedspontaneous emission (ASE). The optical isolator is not the only opticalcomponent which is inserted at the front of doped fiber. Generally, atransmission equipment requires a wavelength demultiplexer for anoptical surveillance signal, a optical coupler for an optical signalmonitor and a wavelength multiplexer for a pumping light. All of theseoptical components have loses. Further, the noise figure of Erbium dopedfiber having a length of 20-30 m is not negligible. Where the noisefigure is defined by the ratio of the S/N ratio on the input side andthe S/N ratio on the output side.

[0007] The optical signal which is attenuated in the transmission pathalso suffers losses due to the optical components. The optical signal isamplified in the EDF of which a noise figure is large. Theabove-described transmission equipment cannot achieve a noise figureless than 6 dB.

[0008] When a non-regenerative multiple amplifying transmission isperformed using k units of optical amplifiers, the S/N ratio degradationamount increases in proportion to the step number k. Accordingly, in anactual optical transmission system in which there exists an upper limitin the total S/N ratio degradation amount, the repeating step numberdecreases as the S/N ratio degradation amount in the optical amplifiersincreases. This eventually shortens the light transmission distance.

[0009] For example, when setting optical amplifiers, the S/N ratiodegradation amount of same are 4 dB, and the S/N ratio degradationamount of others are 6 dB at intervals of 80 km. Under a requirementthat the total S/N ratio deterioration amount can not be more than 12dB, a total S/N ratio degradation amount of the 4 dB optical amplifiersbecomes 12 dB when three steps are repeated, and the total S/N ratiodegradation amount of the 6 dB optical amplifiers becomes 12 dB when twosteps are repeated. Thus, when the 4 dB optical amplifiers are used inthree repeated steps it is possible, thus making it possible to transmita signal light for 240 km. Whereas, when the 6 dB optical amplifiers areused in two repeated steps it is possible to transmit a signal light for160 km.

SUMMARY OF THE INVENTION

[0010] A first object of the present invention is to eliminate theabove-described inconvenience as well as to provide an opticaltransmission device which is applicable to the low-noise opticaltransmission system and is, suppressing a degradation of the S/N ratio,suitable for a long haul optical transmission.

[0011] A second object of the present invention is to provide abi-directional optical transmission system suitable for the longdistance optical transmission.

[0012] In order to solve the above-mentioned problems, a terminalstation repeater or an in-line repeater is configured by at least onebuffer light amplifying unit in contact with a transmission path and atleast one core light amplifying unit in contact with the buffer lightamplifying unit. This configuration allows the buffer light amplifyingunit to amplify an input signal before a signal light, which has beenattenuated because of the propagation along the transmission path,suffers from losses from the optical devices, thereby making it possibleto prevent noise degradation in the optical transmission device.

[0013] By use of the present invention it is possible to embody anoptical transmission device in an optical transmission system, whereindegradation of the S/N ratio is suppressed. Thus, the present inventionis suitable for long haul optical transmission. Further, by employingthe optical transmission device of the present invention it is possibleto develop an optical transmission system suitable for the long distanceoptical transmission.

[0014] The present invention provides an optical transmission devicewhich reduces optical noise in bi-directional transmission systems. Theoptical transmission device includes a core light amplifying unit and afirst buffer light amplifying unit for amplifying a first signal lightfrom a first transmission path and an amplified second signal light fromthe core light amplifying unit. The first buffer light amplifying unitsupplies the core light amplifying unit with the first signal light, andsupplies the first transmission path with the amplified second signallight. A second buffer light amplifying unit is provided for amplifyinga second signal light from a second transmission path and an amplifiedfirst signal light from the core light amplifying unit. The secondbuffer light amplifying unit supplies the core light amplifying unitwith the second signal light, and supplies the second transmission pathwith the amplified first signal light.

[0015] The core light amplifying unit includes a first opticalmultiplexer/demultiplexer, a second optical multiplexer/demultiplexer, afirst optical amplifier for amplifying the first signal light from thefirst optical multiplexer/demultiplexer so as to send out the amplifiedfirst signal light to the second optical multiplexer/demultiplexer, anda second optical amplifier for amplifying the second signal light fromthe second optical multiplexer/demultiplexer so as to send out theamplified second signal light to the first opticalmultiplexer/demultiplexer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The scope of the present invention will be apparent from thefollowing detailed description, when taken in conjunction with theaccompanying drawings, and such detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description, inwhich:

[0017]FIG. 1 is a basic function block diagram illustrating anembodiment of a bi-directional optical transmission system according tothe present invention;

[0018]FIG. 2 is a block diagram illustrating functions of an embodimentof a terminal station repeater according to the present invention;

[0019]FIG. 3 is a block diagram illustrating functions of an embodimentof an intermediate repeater according to the present invention;

[0020]FIG. 4 is a block diagram for illustrating a configuration andfunctions of an embodiment of a terminal station repeater according tothe present invention;

[0021]FIG. 5 is a block diagram illustrating an embodiment of a bufferlight amplifying unit according to the present invention;

[0022]FIG. 6 is a block diagram for illustrating a configuration andfunctions of an embodiment of a control unit according to the presentinvention;

[0023]FIG. 7 is a block diagram for illustrating a configuration andfunctions of an embodiment of a control unit according to the presentinvention;

[0024]FIG. 8 is a diagram illustrating an experimental result obtainedby using an embodiment of a terminal station repeater according to thepresent invention;

[0025]FIG. 9 is a diagram illustrating an experimental result obtainedby using an embodiment of a terminal station repeater according to thepresent invention;

[0026]FIG. 10 is a diagram illustrating an experimental result obtainedby using an embodiment of a terminal station repeater according to thepresent invention;

[0027]FIG. 11 is a block diagram for illustrating a configuration andfunctions of an embodiment of an intermediate repeater according to thepresent invention;

[0028]FIG. 12 is a diagram illustrating another embodiment of anintermediate repeater according to the present invention;

[0029]FIG. 13 is a diagram illustrating another embodiment of anintermediate repeater according to the present invention; and

[0030]FIGS. 14 and 15 are block diagrams illustrating uni-directionaltransmission equipment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Various modes for carrying out the present invention will bedescribed below using the figures.

[0032]FIG. 1 is a basic function block diagram illustrating anembodiment of an optical transmission system according to a first modefor carrying out the present invention. The optical transmission systemincludes optical transmitting units 2 including a plurality of opticaltransmitters 1, optical receiving units 4 including a plurality ofoptical receivers 3, and terminal station repeaters 5. Also, theterminal station repeaters 5 are connected with intermediate repeaters 6through at least one transmission path 7. This configuration allows asignal light to be transmitted bi-directionally from an opticaltransmitting unit 2 ₁ to an optical receiving unit 4 ₂, or from anoptical transmitting unit 2 ₂ to an optical receiving unit 4 ₁.

[0033] A line state of the transmission path 7 is supervised in thefollowing a manner: a supervisory light signal introduced by asupervisory unit 8 is introduced into a transmission path by asupervisory light signal multiplexer/demultiplexer 9, and in a nextterminal station repeater or intermediate repeater, a supervisory lightsignal launched out from a supervisory light signalmultiplexer/demultiplexer 9 is introduced into a supervisory unit 8,thereby monitoring the line state of the transmission path. However, thesupervisory units are not essential, and even if they are removed, thereare no adverse influence or effects on the present invention.

[0034] A transmission path 7 ₁ is connected with buffer light amplifyingunits 10 ₁, at least one of which is set within a terminal stationrepeater 5 ₁. The buffer light amplifying units 10 ₁ are connectedthrough a supervisory light signal multiplexer/demultiplexer 9 ₁ withcore light amplifying units 11 ₁, at least one of which is set withinthe terminal station repeater 5 ₁. Further, the core light amplifyingunits 11 ₁ are connected with an optical multiplexer/demultiplexer 12 ₁,and at least one optical transmitter 1 and optical receiver 3. Aterminal station repeater 5 ₂ is configured in much the same way.

[0035] A transmission path 7 ₁ or 7 ₂ is connected with buffer lightamplifying units 10 ₂ or 10 ₃, at least one of which is respectively setwithin an intermediate repeater 6 ₁. The buffer light amplifying units10 ₂ or 10 ₃ are connected through a supervisory light signalmultiplexer/demultiplexer 9 ₂ or 9 ₃ with core light amplifying units 11₂, at least one of which is set within the intermediate repeater 6 ₁. Anintermediate repeater 6 ₂ is configured in much the same way.

[0036] In the present transmission system, an arbitrary number ofintermediate repeaters 6 may be arranged in series. Additionally, in thepresent configuration, a so-called bi-directional transmission system isassumed, but the similar configuration is also applicable to a unitdirectional transmission system.

[0037] This configuration allows the buffer light amplifying units toamplify an input signal before a signal light, which has been attenuatedbecause of the transmission path loss, suffers from a loss from theoptical devices. Thus, the configuration makes it possible to prevent anoise figure degradation in the whole optical transmission device. As aresult, it becomes possible to embody an optical transmission systemsuitable for a long haul optical transmission.

[0038] The following description is an embodiment of opticaltransmission devices according to a second mode for carrying out thepresent invention as illustrated in FIG. 2. FIG. 2 specificallyillustrates the terminal station repeater 5-1, which is one of thecomponents in the bi-directional optical transmission system illustratedin FIG. 1. In FIG. 2, a signal light from the transmission path 7 ₁ isintroduced into the buffer light amplifying unit 10 ₁. A portion of theintroduced signal light is branched by an optical coupler 13. A branchedsignal light passes through an optical filter 14, which removes asupervisory signal light, and is then detected by an optical detector15. The detected input supervisory signal is transferred to an opticalamplifier 16 within the core light amplifying unit 11 ₁.

[0039] A signal light having passed through the optical coupler 13 ismultiplexed by an optical multiplexer 17 together with a pumping lightfrom a pumping light source 18, and is then introduced into a rareearth-doped optical fiber 19. Because the rare earth-doped optical fiber19 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing throughthe supervisory light signal multiplexer/demultiplexer 9 ₁, isintroduced into an optical multiplexer/demultiplexer 20 within the corelight amplifying unit 11 ₁. Then, after the signal light is introducedso as to be further amplified into the optical amplifier 16 by way ofthe optical multiplexer/demultiplexer 20, a portion of the signal lightis branched by an optical coupler 22 through an opticalmultiplexer/demultiplexer 21. A branched signal light is detected by anoptical detector 23, and, as an input supervisory unit signal, istransferred to the optical amplifier 16 within the core light amplifyingunit 11 ₁. A signal light having passed through the optical coupler 22reaches the optical multiplexer/demultiplexer 12 ₁. In the opticalmultiplexer/demultiplexer 12 ₁, the signal light is demultiplexed at apredetermined wavelength, reaching the optical receiving unit 4 ₁ (notillustrated).

[0040] Incidentally, the optical couplers 13, 22, the optical detectors23, 15, and the optical filter 14 are not necessarily situated at thesepositions. For example, a plurality of them may be set for everytransmission path at a step next to the optical amplifier 16, or at astep next to the optical multiplexer/demultiplexer 12 ₁.

[0041] A signal light in the reverse direction on the side of theoptical transmitting unit 2 ₁ (not illustrated), after being multiplexedby the optical multiplexer/demultiplexer 12 ₁ within the terminalstation repeater 5 ₁, a portion of the signal light is branched by theoptical coupler 22 within the core light amplifying unit 11 ₁. Abranched signal light is detected by an optical detector 24, and, as aninput supervisory unit signal, is transferred to an optical amplifier 25within the core light amplifying unit 11 ₁.

[0042] A signal light having passed through the optical coupler 22 isamplified through the optical multiplexer/demultiplexer 21 by theoptical amplifier 25. The amplified signal light arrives at thesupervisory light signal multiplexer/demultiplexer 9 ₁ by way of theoptical multiplexer 20. Then, after passing through the supervisorylight signal multiplexer/demultiplexer 9 ₁, the signal is introduced, soas to be further amplified, into the rare earth-doped optical fiber 19within the buffer light amplifying unit 10 ₁ in a direction opposite tothat of the above-mentioned signal light. The amplified signal lightpasses through the optical multiplexer 17, and then a portion thereof isbranched by the optical coupler 13. A branched signal light, passingthrough an optical filter 26 for removing a supervisory unit signallight, is detected by an optical detector 27, and, as an inputsupervisory unit signal, is transferred to an optical amplifier 25within the core light amplifying unit 111. A signal light having passedthe optical coupler 13 is configured to be conveyed into thetransmission path 7 ₁.

[0043] Here, the optical coupler 22 and the optical detector 24 are notnecessarily situated at these positions. For example, a plurality ofthem may be set for every transmission path at a step previous to theoptical amplifier 25, or at a step previous to the optical multiplexer12 ₁. Also, the optical coupler 13, the optical filter 26, and theoptical detector 27 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 25, ata step previous to the buffer light amplifying unit 10 ₁, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₁.

[0044] Meanwhile, a supervisory light signal on the side of asupervisory light signal source (not illustrated), which is introducedby the supervisory light signal multiplexer/demultiplexer 9 ₁ andpassing through the buffer light amplifying unit 10 ₁, is introducedinto the transmission path. Additionally, the optical amplifiers 16, 25are configured to be controlled by the input supervisory unit signal andthe output supervisory unit signal.

[0045] In a terminal station repeater based on the conventionalapparatus, there exist optical losses caused by optical devices such asthe optical multiplexer 20 set at a step previous to the opticalamplifier 16, the supervisory light signal multiplexer/demultiplexer 9₁, and optical isolators set within the optical amplifier 16. Theoptical losses resulted in a factor of bringing about a noise figuredegradation in the whole terminal station repeater. In the presentinvention, however, the buffer light amplifying unit 10 ₁ is configuredto amplify an input signal before an attenuated signal light from thetransmission path 7 ₁ suffers from the losses due to the opticaldevices. Thus, the present invention makes it possible to prevent thenoise figure degradation in the whole terminal station repeater.

[0046] At the same time, according to the buffer light amplifying unit10 ₁ in the present configuration, it becomes unnecessary to employ theoptical isolators, since the buffer amplifier is of low gain. theoptical isolators were essential to an optical amplifier in theconventional apparatus. This makes it possible to prevent a noise figuredegradation in the buffer light amplifying unit 10 ₁ itself, andeventually makes it possible to prevent the noise figure degradation inthe whole terminal station repeater 5 ₁.

[0047] Described below using FIG. 3 is another embodiment of opticaltransmission devices according to a second mode for carrying out thepresent invention:

[0048]FIG. 3 is a configuration diagram illustrating the intermediaterepeater 6 ₁, which is one of the components in the bi-directionaloptical transmission system indicated in FIG. 1. A signal light from thetransmission path 7 ₁ is introduced into the buffer light amplifyingunit 10 ₂. A portion of the introduced signal light is branched by anoptical coupler 28. A branched signal light passes through an opticalfilter 29 for removing a supervisory unit signal light, and is thendetected by an optical detector 30. The detected input supervisory unitsignal is transferred to an optical amplifier 31 within a core lightamplifying unit 11 ₂.

[0049] A signal light having passed through the optical coupler 28 ismultiplexed by an optical multiplexer 32 together with a pumping lightfrom a pumping light source 33, and is then introduced into a rareearth-doped optical fiber 34. Because the rare earth-doped optical fiber34 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing through asupervisory light signal multiplexer/demultiplexer 9 ₂, is introducedinto an optical multiplexer/demultiplexer 35 within the core lightamplifying unit 11 ₂. Then, after being introduced so as to be furtheramplified into an optical amplifier 36 by the opticalmultiplexer/demultiplexer 35, the signal light passes through asupervisory light signal multiplexer/demultiplexer 9 ₃ by way of anoptical multiplexer/demultiplexer 37 and is introduced into a rareearth-doped optical fiber 38 within a buffer light amplifying unit 10 ₃.

[0050] The rare earth-doped optical fiber 38, into which an opticalmultiplexer 40 multiplexes and introduces a pumping light from a pumpinglight source 39, lies in an excited state. Consequently, the signallight is amplified and, passing through the optical multiplexer 40, aportion thereof is branched by an optical coupler 41. A branched signallight, passing through an optical filter 42 for removing a supervisoryunit signal light, is detected by an optical detector 43, and, as anoutput supervisory unit signal, is transferred into the opticalamplifier 36 within the core light amplifying unit 11 ₂. A signal lighthaving passed through the optical coupler 41 is conveyed into atransmission path 7 ₂.

[0051] However, the optical coupler 41, the optical filter 42, and theoptical detector 43 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 36, ata step previous to the buffer light amplifying unit 10 ₃, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₃.

[0052] A signal light in the reverse direction on the side of thetransmission path 7 ₂ is introduced into a buffer light amplifying unit10 ₃. A portion of the introduced signal light is branched by theoptical coupler 41. A branched signal light, passing through an opticalfilter 44 for removing a supervisory unit signal light, is detected byan optical detector 45. The detected input supervisory unit signal istransferred into the optical amplifier 31 within the core lightamplifying unit 11 ₂.

[0053] A signal light having passed through the optical coupler 41 ismultiplexed by the optical multiplexer 40 together with a pumping lightfrom the pumping light source 39, and is then introduced into a rareearth-doped optical fiber 38. Because the rare earth-doped optical fiber38 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing throughthe supervisory light signal multiplexer/demultiplexer 9 ₃, isintroduced into an optical multiplexer/demultiplexer 37 within the corelight amplifying unit 11 ₂. Then, after being introduced so as to befurther amplified into an optical amplifier 31 by the opticalmultiplexer/demultiplexer 37, the signal light passes through thesupervisory light signal multiplexer/demultiplexer 9 ₂ by way of theoptical multiplexer/demultiplexer 35 and is introduced into the rareearth-doped optical fiber 34 within the buffer light amplifying unit 10₂. The rare earth-doped optical fiber 34, into which an opticalmultiplexer 32 multiplexes and introduces a pumping light from thepumping light source 33, lies in an excited state. Consequently, thesignal light is amplified and, passing through the optical multiplexer32, a portion thereof is branched by the optical coupler 28. A branchedsignal light, passing through an optical filter 46 for removing asupervisory unit signal light, is detected by an optical detector 47,and, as an output supervisory unit signal, is transferred into theoptical amplifier 31 within the core light amplifying unit 11 ₂. Asignal light having passed through the optical coupler 28 is conveyedinto the transmission path 7 ₁.

[0054] However, the optical coupler 28, the optical filter 46, and theoptical detector 47 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 31, ata step previous to the buffer light amplifying unit 10 ₂, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₂.

[0055] Meanwhile, a supervisory light signal on the side of asupervisory light signal source (not illustrated), which is introducedby the supervisory light signal multiplexers/demultiplexers 9 ₂, 9 ₃and, passing through the buffer light amplifying units 10 ₂,10 ₃, isintroduced into the transmission paths 7 ₁, 7 ₂. In addition, theoptical amplifiers 31, 36 are configured to be controlled by the inputsupervisory unit signal and the output supervisory unit signal.

[0056] In a terminal station repeater based on the conventionalapparatus, there exist optical losses caused by optical devices such asthe optical multiplexers 35, 37 set at a step previous to the opticalamplifiers 31, 36, the supervisory light signalmultiplexers/demultiplexers 9 ₂, 9 ₃, and optical isolators set withinthe optical amplifiers 31, 36. The optical losses resulted in a factorof bringing about a noise figure degradation in the whole terminalstation repeater. In the present invention, however, the buffer lightamplifying unit 10 ₂, or 10 ₃ is configured to amplify an input signalbefore an attenuated signal light from the transmission paths 7 ₁, 7 ₂suffers from the losses due to the optical devices, thus making itpossible to prevent the noise figure degradation in the whole terminalstation repeater.

[0057] At the same time, according to the buffer light amplifying units10 ₂, 10 ₃ in the present configuration, it becomes unnecessary toemploy the optical isolators, which were essential to an opticalamplifier in the prior art. This makes it possible to prevent a noisefigure degradation in the buffer light amplifying units 10 ₂, 10 ₃themselves, too, and eventually makes it possible to prevent the noisefigure degradation in the whole terminal station repeater 6 ₁.

[0058] Described below using FIG. 4 is still another embodiment ofoptical transmission devices according to a second mode for carrying outthe present invention.

[0059] Here, FIG. 4 is a block diagram illustrating functions of aterminal station repeater. FIG. 5 is a block diagram illustrating abuffer light amplifying unit, and FIG. 6 is a block diagram illustratingfunctions of a control unit. Also, FIG. 8, and FIG. 9 or FIG. 10 arediagrams indicating experimental results obtained by using a terminalstation repeater.

[0060] In FIG. 4, the signal light includes the four wavelengths:λ1=1530.33 nm, λ2=1531.90 nm, λ3=1533.47 nm, and λ4=1535.04 nm.Moreover, a probe light is transmitted by a probe light source 48 with awavelength of λp1=1543.73. Meanwhile, the four wavelengths are received:λ5=1555.75 nm, λ6=1557.36 nm, λ7=1558.98 nm, and λ8=1560.61 nm.Furthermore, a probe light is received by a probe light receiver (notillustrated) with a wavelength of λp2=1546.92.

[0061] Each of the wavelengths of λ1 to λ4 is branched by opticalcouplers 22-1 to 22-4 each of the branching ratios of which is 5:95, andis respectively detected by optical detectors 24 ₁ to 24 ₄. An inputsupervisory unit signal for each of the detected wavelengths istransferred into a control unit 49 described hereinafter inside anoptical amplifier 25. The signal lights and the probe light, which havepassed through optical couplers 22 ₁ to 22 ₄, are multiplexed by anoptical multiplexer 50 inside an optical multiplexer/demultiplexer 12 ₁,and passes through a dispersion compensator 51 inside an opticalamplifier 25. The dispersion compensator 51 compensates dispersioncharacteristics which a signal light causes when passing throughtransmission path 7 ₁ to 7 ₄. The multiplexed lights having passedthrough the dispersion compensator 51 pass through an optical isolator52, then being introduced into a rare earth-doped optical fiber 53.

[0062] The rare earth-doped optical fiber 53 is in an excited state,since a pumping light has been introduced therein through an opticalmultiplexer 55 by a pumping light source 54, which is a semiconductorlaser having the oscillation wavelength in proximity to 1480 nm.Accordingly, the multiplexed lights are amplified, and passing throughan optical isolator 56, an optical multiplexer 20, and the supervisorylight signal multiplexer/demultiplexer 9 ₁, they are introduced into abuffer light amplifying unit 10 ₁. The supervisory light signalmultiplexer/demultiplexer 9 ₁ multiplexes the supervisory light signalat 1.48 μm wavelength and the signal lights.

[0063] The multiplexed lights introduced into the buffer lightamplifying unit 10 ₁ is introduced into an erbium-doped optical fiber asa rare earth-doped optical fiber 19, into which a pumping light has beenintroduced through an optical multiplexer 17 from a semiconductor laser(a pumping light source 18) having the oscillation wavelength inproximity to 980 nm. Although the erbium-doped optical fiber 19 is beingin an exited state, the lights which can be amplified are themultiplexed lights at λ1 to λ4 wavelengths and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss. Also, the pumping light source 18 is monitoredby an optical detector 57 for detecting a portion of the optical outputfrom the pumping light source 18. At that time, a control unit 58 isconfigured to control the devices so that the pumping light sourcesupervisory unit signal remains unchanged.

[0064] The amplified multiplexed lights and the supervisory light signalat 1.48 μm are partially branched by an optical coupler 13 the branchingratio of which is 5:95. A portion of the probe light, which has passedthrough a narrow bandwidth optical filter 26 allowing the probe light topass through, is detected by an optical detector 27. The detected probelight supervisory unit signal is conveyed to the above-mentioned controlunit 49. The control unit 49 is configured to control the pumping lightsource 54 so that the probe light supervisory unit signal remainsunchanged. In this way, by controlling the devices so that the probelight supervisory unit signal remains unchanged, it becomes possible tocontrol and maintain all the signal lights at λ1 to λ4 wavelengths atfixed outputs.

[0065] Namely, if any of the signal lights at λ1 to λ4 wavelengths shutdown, or even if a signal light other than the signal lights at λ1 to λ4wavelengths is newly added, no influences are exerted on optical outputsof the signal lights at λ1 to λ4 wavelengths (for example, when a signallight at λ4 is cut off, signal lights at λ1 to λ3). This always makes itpossible to perform a fixed and stable control of the device.

[0066] The amplified multiplexed lights and the supervisory light signalhaving passed through the optical coupler 13 are transferred to thetransmission path 7 ₁, which is a single mode transmission fiber.

[0067] Here, the dispersion compensator 51 may be omitted whendispersion characteristics of the transmission paths exerts no influenceon transmission characteristics of the whole system. Also, a place atwhich the dispersion compensator 51 is to be set does not necessarilycoincide with this position. A part consisting of the rare earth-dopedoptical fiber 53, the pumping light source 54, and the opticalmultiplexer 55 may be replaced by a semiconductor optical amplifier. Inthis case, it is advisable that an amplification ratio is controlled bya pumping electric current instead of the pumping light source 54. As ismuch the same way, a part consisting of the rare earth-doped opticalfiber 19, the pumping light source 18, and the optical multiplexer 17may be replaced by a semiconductor optical amplifier.

[0068] On the other hand, reverse-directional signal lights at λ5 to λ8and a reverse-directional multiplexed light at λp2, which aretransmitted from the transmission fiber 7 ₁, and the supervisory lightsignal at 1.48 μm are partially branched by the optical coupler 13 thebranching ratio of which is 5:95. A portion of the probe light, whichhas passed through a narrow bandwidth optical filter 14 allowing theprobe light to pass through, is detected by an optical detector 15. Thedetected input supervisory unit signal is conveyed to a control unit 59described hereinafter. The multiplexed lights and the supervisory lightsignal having passed through the optical coupler 13 are multiplexed witha pumping light from the semiconductor laser as the pumping light source18 having the oscillation wavelength in proximity to 980 nm by theoptical multiplexer 17 inside the buffer light amplifying unit 10 ₁,thus being amplified by the erbium-doped optical fiber 19. In this case,too, the lights which can be amplified are the multiplexed lights at λ5to λ8 and at λp2 only. The supervisory light signal at 1.48 μm passesthrough the fiber after suffering from some loss. The supervisory lightsignal at 1.48 μm having passed through the fiber is demultiplexed bythe supervisory light signal multiplexer/demultiplexer 9 ₁, then beingtransmitted into a supervisory light signal path. The multiplexedlights, having passed through the optical multiplexer 20 and an opticalisolator 60 inside the optical amplifier 16, are multiplexed by anoptical multiplexer 62 with a pumping light from a semiconductor laseras a pumping light source 61 having the oscillation wavelength inproximity to 980 nm, thus being amplified by an erbium-doped opticalfiber 63. Also, the pumping light source 61 is monitored by an opticaldetector 64 for detecting a portion of the optical output from thepumping light source 61. At that time, a control unit 65 is configuredto control the devices so that the pumping light source supervisory unitsignal remains unchanged.

[0069] The amplified signal lights, passing through an optical isolator66, are introduced into a dispersion compensator 67. After beingamplified by a second erbium-doped optical fiber 68, the signal lightspass through an optical multiplexer 69, then being outputted from anoptical isolator 70. The second erbium-doped optical fiber 68 is in anexited state, since it is multiplexed with a pumping light from asemiconductor laser (a second pumping light source 71) having theoscillation wavelength in proximity to 980 nm. The multiplexed lightsfrom the optical isolator 70 are partially branched by an opticalcoupler 72 the branching ratio of which is 5:95. Branched multiplexedlights pass through a narrow bandwidth optical filter 73 allowing theprobe light to pass through, and a portion of the probe light isdetected by an optical detector 74. The detected output supervisory unitsignal is conveyed to the control unit 59 inside the optical amplifier16. At that time, a pumping light source 71 is configured to becontrolled so that the output supervisory unit signal remains unchanged.

[0070] Multiplexed lights having passed through the optical coupler 72are demultiplexed for each of the wavelengths of λ5 to λ8 by an opticaldemultiplexer 75. The each wavelength is branched by optical couplers 22₅ to 22 ₈ each of the branching ratios of which is 5:95, and isrespectively detected by optical detectors 23 ₁ to 23 ₄. An outputsupervisory unit signal for each of the detected wavelengths istransferred into the control unit 59 inside the optical amplifier 16. Asignal light at each of the wavelengths having passed through theoptical couplers 22 ₅ to 22 ₈ is conveyed to a terminal station unit(not illustrated).

[0071] In the present configuration, a signal input power into thebuffer light amplifying unit 10 ₁ from the transmission path 7 ₁ fallsin a range of −30 dBm to −5 dBm, and a signal amplification gain in thebuffer light amplifying unit 10 ₁ is equal to an order of about 10 dB.Since there is furnished no optical isolator within the buffer lightamplifying unit 10 ₁, attention must be paid to oscillation phenomena oflight. Accordingly, the signal amplification gain in the buffer lightamplifying unit 10 ₁ should be, preferably, 30 dB or less, or morepreferably, 15 dB or less. Also, by making a positive gain the signalamplification gain in the buffer light amplifying unit 10 ₁, a noisefigure for a signal input from the transmission path 7 ₁ in the terminalstation repeater is obviously improved as compared with the methods inthe prior art, but more preferably, it should be 5 dB or more.

[0072] Moreover, it is preferable that amplification gain distributionsin the core light amplifying unit 11 ₁ and the buffer light amplifyingunit 10 ₁ should be calculated from a necessary output power into thetransmission path 7 ₁. For example, assuming that the output power intothe transmission path is equal to +11 dBm per signal wavelength, thetotal signal power (λ1 to λ4 and λp2) turns out to be +18 dBm, andconsequently it is preferable that a power of the pumping light source18 should be set to be about 1.25 to 3.3 times as high as this power.When the power of the pumping light source is not enough, as illustratedin FIG. 5, the following units may be added, thereby providing abi-directional pumping for the erbium-doped optical fiber: a new pumpinglight source 18-a, an optical detector 57-a for detecting the opticaloutput thereof, a control unit 58-a for keeping a detected supervisoryunit signal unchanged, and an optical multiplexer 17-a for introducingthe pumping light. Besides, in any case, it is preferable that thepumping light source 18, which corresponds to a forward pumping for themultiplexed lights from the transmission path 7 ₁, is furnished.

[0073] Based on the ability of the pumping light source 18 set above, itis possible to set an input power of the multiplexed lights, which areconveyed into the buffer light amplifying unit 10 ₁ from the opticalamplifier 25, at the value of [the optical output from the buffer lightamplifying unit 10-1 (+11 dBm)−X dB]. It is preferable that a range of Xshould be 0 to 20. An adjustment of X makes it possible to set, at theabove-mentioned more preferable value, a signal amplification gain for asignal input power which is reverse-directional, i.e. in a directionfrom the transmission path 7 ₁.

[0074] Here, a 980 nm semiconductor laser may be employed as the pumpinglight source 54 inside the optical amplifier 25. Also, a 1480 nmsemiconductor laser may be employed as the pumping light source 71inside the optical amplifier 16.

[0075] However, an employment of the 980 nm semiconductor laser is bestsuited for the pumping light source 18 inside the buffer lightamplifying unit 10 ₁ and the pumping light source 61 ₁ inside theoptical amplifier 16.

[0076] Described below with reference to FIG. 6 is a configuration of anembodiment of the control unit 49.

[0077] An output supervisory unit signal transmitted into the controlunit 49 has been compared with a predetermined reference value 77 by acomparing unit 76. A pumping light source 54 (not illustrated) iscontrolled by transmitting an error signal relative to the referencevalue 77 to a driving circuit 78.

[0078] Also, input supervisory unit signals corresponding to λ1 to λ4transmitted into the control unit 49 have been respectively comparedwith a reference value 79 by a comparing unit 80. When they are higherthan the predetermined value, a normal signal is transmitted to awavelength number detection circuit 81, and when they are lower, anabnormal signal is transmitted. The wavelength number detection circuit81 counts the wavelength number of the transmitted normal signal, thusjudging the wavelength number which can be transferred at the moment.When there turns out to be no wavelength which can be transferred, thewavelength number detection circuit issues an alarm. Also, at that time,the alarm is transmitted to the driving circuit 78, too. Having receivedthe alarm, the driving circuit 78 is configured to control and halt thepumping light source 54 (not illustrated).

[0079] Another configuration of an embodiment of the control unit 59will be described hereunder and illustrated in FIG. 7.

[0080] An output supervisory unit signal transmitted into the controlunit 59 has been compared with a predetermined reference value 83 by acomparing unit 82. The pumping light source 71 (not illustrated) iscontrolled by transmitting an error signal relative to the referencevalue 83 to a driving circuit 84.

[0081] Output supervisory unit signals corresponding to λ5 to λ8 and aninput supervisory unit signal corresponding to λp2 transmitted into thecontrol unit 59 have been respectively compared with a reference value85 by a comparing unit 86. When they are higher than the predeterminedvalue, a normal signal is transmitted to a wavelength number detectioncircuit 87, and when they are lower, an abnormal signal is transmitted.The wavelength number detection circuit 87 counts the wavelength numberof the transmitted normal signal, thus judging the wavelength numberwhich can be transferred at the moment. When there turns out to be nowavelength which can be transferred, the wavelength number detectioncircuit issues an alarm. Also, at that time, the alarm is transmitted tothe driving circuit 84, too. Having received the alarm, the drivingcircuit 84 is configured to control and halt the pumping light source 71(not illustrated). The control may be executed so that the alarm isissued even when the signal which can be transferred is the onecorresponding to λp2 only.

[0082] A characteristic in the buffer light amplifying unit 10 ₁according to the present invention is to introduce multiplexed lightsinto the rare earth-doped optical fiber 19 from bi-directions and thenamplify the multiplexed lights bi-directionally. Also, anothercharacteristic is to amplify attenuated multiplexed lights introducedfrom the transmission path 7 ₁ before they suffer from considerablelosses.

[0083] As described above, the control unit 58 controls and maintains anoutput of the pumping light source 18 at a fixed value. This makes itpossible to allow the buffer light amplifying unit 10 ₁ to function as alight amplifying unit having an approximately constant gain as well asto maintain a stable and lowered noise figure of the buffer lightamplifying unit. Controlling an output of the pumping light source 18 ata fixed value is important for reducing a wavelength dependence of thegain, which rare earth-doped optical fibers generally have, and forsuppressing a wavelength deviation variation of the gain, which turnsout to be a problem in the transmission characteristics.

[0084] Furthermore, as described above, the control unit 49 controls andmaintains an output of the probe light at a fixed value. This makes itpossible to automatically control outputs of multiplexed lights into thetransmission path 7 ₁ inside the buffer light amplifying unit 10 ₁.

[0085]FIGS. 8, 9 and 10 illustrate the results of an experiment in whichthe input/output characteristics and a noise figure of the signal lightare measured in the present embodiment when operated. More particularly,FIGS. 8 and 9 illustrate the results of the experiment on the wholesystem including a buffer light amplifier and a core light amplifier.FIG. 10 illustrates the results of the experiment on only the bufferlight amplifier.

[0086] The measurement points of the experiment are explained by usingFIG. 4. The input power of FIG. 8 is the signal level from transmissionline 17 ₁. The output power of FIG. 8 is the signal level after anamplification of the first stage of optical amplifier 16. ASE LEVEL isthe value that divided the power of amplified spontaneous emission light(ASE) of the 1550 nm by the wavelength. The ASE level is used for thecalculation of the noise figure. This experiment is implemented usingthe following conditions: (a) the reverse signal of about +11 dBm, thatis amplified by the optical amplifier 25, is introduced to the bufferlight amplifier 10 ₁; and (b) signal output of +17 dBm is alwaysdelivered to the transmission line 17 ₁. Under the above conditions, theoutput power of the pumping light source for the buffer light amplifieris supplied at about 110 mW, that is the equivalent of 2.2 times of +17dBm, which is signal output power.

[0087] According to FIG. 9, which illustrates a graph of the gain andnoise figure against the input power, it is clear that noise figureagainst the input signal from the transmission path is controlled to 3.9dB or less. Even if the temperature fluctuation is a non-experimentalsystem and dispersion at the time of manufacturing are considered,according to the present invention, it can be controlled to 4.5 dB orless. In addition, it can be made less than 4.0 dB by maintainingexperimental structure in the actual system.

[0088] The excitation power of semiconductor laser 61 ₁ of the pumpinglight source of the first stage of optical amplifier 16 was made 75 mWin this experiment. Therefore, output power cannot be constantlycontrolled. However, it is possible to constantly control output powerby making the excitation power of a semiconductor laser 61 ₁ 100-120 mW.

[0089]FIG. 10 illustrates that the input signal gain of the buffer lightamplifier 10 ₁ maintains 10 dB constantly under the condition of signaloutput of reverse direction that is maintained at +17 dBm.

[0090] Described below using FIG. 11 is an even further embodiment ofoptical transmission devices according to a second mode for carrying outthe present invention. FIG. 11 is a block diagram which illustrates aconfiguration and functions of an intermediate repeater.

[0091] Signal lights at the four wavelengths are transmitted from atransmission path 7-1: λ1=1530.33 nm, λ2=1531.90 nm, λ3=1533.47 nm, andλ4=1535.04 nm. Moreover, a probe light at λp1=1543.73 is transmitted.Meanwhile, the four wavelengths are transmitted from a transmission path7 ₂: λ5=1555.75 nm, λ6=1557.36 nm, λ7=1558.98 nm, and λ8=1560.61 nm.Furthermore, a probe light at λp2=1546.92 is transmitted.

[0092] The present embodiment differs from the terminal station repeaterillustrated in that Buffer light amplifying units 10 ₂, 10 ₃ are set atthe both ends of a core light amplifying unit 11 ₂, and the core lightamplifying unit 11 ₂ is configured by two units of optical amplifiers31, 36, each of which has the same configuration as that of the opticalamplifier 16 in FIG. 4.

[0093] The signal lights at λ1 to λ4 and a multiplexed light at λp1,which are transmitted from the transmission fiber 7 ₁, and a supervisorylight signal at 1.48 μm are partially branched by an optical coupler 28the branching ratio of which is 5:95. A portion of the probe light,which has passed through a narrow bandwidth optical filter 29 allowing aprobe light to pass through, is detected by an optical detector 30. Thedetected input supervisory unit signal is conveyed to a control unit 59₂ described hereinafter. The multiplexed lights and the supervisorylight signal having passed through the optical coupler 28 aremultiplexed with a pumping light from a semiconductor laser as a pumpinglight source 33 having the oscillation wavelength in proximity to 980 nmby an optical multiplexer 32 inside the buffer light amplifying unit 10₂, thus being amplified by an erbium-doped optical fiber as a rareearth-doped optical fiber 34. At that time, the erbium-doped opticalfiber 34 is in an exited state, but the lights which can be amplifiedare the multiplexed lights at λ1 to λ4 and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss.

[0094] Also, the pumping light source 33 is monitored by an opticaldetector 57 ₂ for detecting a portion of the optical output from thepumping light source 33. At that time, a control unit 58 ₂ is configuredto control the devices so that the pumping light source supervisory unitsignal remains unchanged.

[0095] The supervisory light signal at 1.48 μm having passed through thefiber is demultiplexed by a supervisory light signalmultiplexer/demultiplexer 9 ₂, then being transmitted into a supervisorylight signal path. The multiplexed lights, having passed through anoptical multiplexer 35 and an optical isolator 60 ₂ inside the opticalamplifier 36, are multiplexed by an optical multiplexer 62 ₂ with apumping light from a semiconductor laser as a pumping light source 61-2having the oscillation wavelength in proximity to 980 nm, thus beingamplified by an erbium-doped optical fiber 63 ₂. Also, the pumping lightsource 61 ₂ is monitored by an optical detector 64 ₂ for detecting aportion of the optical output from the pumping light source 61 ₂. Atthat time, a control unit 65 ₂ is configured to control the devices sothat the pumping light source supervisory unit signal remains unchanged.

[0096] The amplified signal lights, passing through an optical isolator66 ₂, are introduced into a dispersion compensator 67 ₂. After beingamplified by a second erbium-doped optical fiber 68 ₂, the signal lightspass through an optical multiplexer 69 ₂, then being outputted from anoptical isolator 70 ₂. Since it is multiplexed with a pumping light froma semiconductor laser as a second pumping light source 71 ₂ having theoscillation wavelength in proximity to 980 nm, the second erbium-dopedoptical fiber 68 ₂ is in an exited state. The multiplexed lights fromthe optical isolator 70 ₂, passing through an optical multiplexer 37 anda supervisory light signal multiplexer/demultiplexer 9 ₃, are introducedinto the buffer light amplifying unit 10 ₃. The supervisory light signalmultiplexer/demultiplexer 9 ₃ multiplexes the supervisory light signalat 1.48 μm wavelength and the signal lights.

[0097] The multiplexed lights introduced into the buffer lightamplifying unit 10 ₃ is introduced into an erbium-doped optical fiber asa rare earth-doped optical fiber 38, into which a pumping light has beenintroduced through an optical multiplexer 40 from a semiconductor laseras a pumping light source 39 having the oscillation wavelength inproximity to 980 nm. Although the erbium-doped optical fiber 38 is beingin an exited state, the lights which can be amplified are themultiplexed lights at λ1 to λ4 wavelengths and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss.

[0098] Also, the pumping light source 39 is monitored by an opticaldetector 57 ₃ for detecting a portion of the optical output from thepumping light source 39. At that time, a control unit 58 ₃ is configuredto control the devices so that the pumping light source supervisory unitsignal remains unchanged. The amplified multiplexed lights and thesupervisory light signal at 1.48 μm are partially branched by an opticalcoupler 41 the branching ratio of which is 5:95. A portion of the probelight, which has passed through a narrow bandwidth optical filter 42allowing a probe light to pass through, is detected by an opticaldetector 43. The detected probe light supervisory unit signal isconveyed to the above-mentioned control unit 59 ₂. The control unit 59 ₂is configured to control the pumping light source 71 ₂ so that the probelight supervisory unit signal remains unchanged. In this way, bycontrolling the devices so that the probe light supervisory unit signalremains unchanged, it becomes possible to control and maintain all thesignal lights at λ1 to λ4 wavelengths at fixed outputs.

[0099] If any of the signal lights at λ1 to λ4 wavelengths is cut off,or even if a signal light other than the signal lights at λ1 to λ4wavelengths is newly added, no influences are exerted on optical outputsof the signal lights at λ1 to λ4 wavelengths (for example, when a signallight at λ4 is cut off, signal lights at λ1 to λ3). This always makes itpossible to perform a fixed and stable control of the device.

[0100] The amplified multiplexed lights and the supervisory light signalhaving passed through the optical coupler 41 are transferred to thetransmission path 7-2, which is a single mode transmission fiber.

[0101] A part including the rare earth-doped optical fibers 34, 63 ₂, 68₂, 38, the pumping light sources 33, 61 ₂, 71 ₂, 39, and the opticalmultiplexers 32, 62 ₂, 40 may be replaced by a semiconductor opticalamplifier. In this case, it is advisable that an amplification ratio iscontrolled by a pumping electric current instead of the pumping lightsources 33, 61 ₂, 71 ₂, 39.

[0102] On the other hand, reverse-directional signal lights at λ5 to λ8and a reverse-directional multiplexed light at λ2, which are transmittedfrom the transmission fiber 7 ₂, and a supervisory light signal at 1.48μm are partially branched by the optical coupler 41 the branching ratioof which is 5:95. The wavelength of the supervisory light signal canalso be 1.51 μm. A portion of the probe light, which has passed througha narrow bandwidth optical filter 44 allowing a probe light to passthrough, is detected by an optical detector 45. The detected inputsupervisory unit signal is conveyed to a control unit 59 ₃ describedhereinafter.

[0103] The multiplexed lights and the supervisory light signal havingpassed through the optical coupler 41 are multiplexed with a pumpinglight from the semiconductor laser as the pumping light source 39 havingthe oscillation wavelength in proximity to 980 nm by the opticalmultiplexer 40 inside the buffer light amplifying unit 10 ₃, thus beingamplified by the erbium-doped optical fiber as the rare earth-dopedoptical fiber 38. At that time, the erbium-doped optical fiber 38 is inan exited state, but the lights which can be amplified are themultiplexed lights at λ5 to λ8 and the probe light only. The supervisorylight signal at 1.48 μm passes through the fiber after suffering fromsome loss.

[0104] Also, the pumping light source 39 is monitored by the opticaldetector 57 ₃ for detecting a portion of the optical output from thepumping light source 39. At that time, a control unit 58 ₃ is configuredto control the devices so that the pumping light source supervisory unitsignal remains unchanged. The supervisory light signal at 1.48 μm havingpassed through the fiber is demultiplexed by the supervisory lightsignal multiplexer/demultiplexer 9 ₃, then being transmitted into asupervisory light signal path. The multiplexed lights, having passedthrough the optical multiplexer 37 and an optical isolator 60 ₃ insidethe optical amplifier 31, are multiplexed by an optical multiplexer 62 ₃with a pumping light from a semiconductor laser as a pumping lightsource 61 ₃ having the oscillation wavelength in proximity to 980 nm,thus being amplified by an erbium-doped optical fiber 63 ₃. Also, thepumping light source 61 ₃ is monitored by an optical detector 64 ₃ fordetecting a portion of the optical output from the pumping light source61 ₃. At that time, a control unit 65 ₃ is configured to control thedevices so that the pumping light source supervisory unit signal remainsunchanged.

[0105] The amplified signal lights, passing through an optical isolator66 ₃, are introduced into a dispersion compensator 67 ₃. After beingamplified by a second erbium-doped optical fiber 68 ₃, the signal lightspass through an optical multiplexer 69 ₃, then being outputted from anoptical isolator 70 ₃. The second erbium-doped optical fiber 68 ₃ is inan exited state, since it is multiplexed with a pumping light from asemiconductor laser as a second pumping light source 71 ₃ having theoscillation wavelength in proximity to 980 nm. The multiplexed lightsfrom the optical isolator 70 ₃, passing through an optical multiplexer35 and the supervisory light signal multiplexer/demultiplexer 9 ₂, areintroduced into the buffer light amplifying unit 10 ₂. The supervisorylight signal multiplexer/demultiplexer 9 ₂ multiplexes the supervisorylight signal at 1.48 μm wavelength and the signal lights.

[0106] The multiplexed lights introduced into the buffer lightamplifying unit 10 ₂ is introduced into the erbium-doped optical fiberas the rare earth-doped optical fiber 34, which is raised to be in anexited state by a pumping light from the semiconductor laser as thepumping light source 33 having the oscillation wavelength in proximityto 980 nm. The lights which can be amplified are the multiplexed lightsat λ5 to λ8 wavelengths and the probe light only. The supervisory lightsignal at 1.48 μm passes through the fiber after suffering from someloss.

[0107] Also, the pumping light source 33 is monitored by the opticaldetector 57 ₂ for detecting a portion of the optical output from thepumping light source 33. At that time, the control unit 58 ₂ isconfigured to control the devices so that the pumping light sourcesupervisory unit signal remains unchanged. The amplified multiplexedlights and the supervisory light signal at 1.48 μm are partiallybranched by the optical coupler 28 the branching ratio of which is 5:95.A portion of the probe light, which has passed through a narrowbandwidth optical filter 46 allowing a probe light to pass through, isdetected by an optical detector 47.

[0108] The detected probe light supervisory unit signal is conveyed tothe above-mentioned control unit 59 ₃. The control unit 59 ₃ isconfigured to control the pumping light source 71 ₃ so that the probelight supervisory unit signal remains unchanged. In this way, bycontrolling the devices so that the probe light supervisory unit signalremains unchanged, it becomes possible to control and maintain all thesignal lights at λ5 to λ8 wavelengths at fixed outputs. If any of thesignal lights at λ5 to λ8 wavelengths is cut off, or even if a signallight other than the signal lights at λ5 to λ8 wavelengths is newlyadded, no influences are exerted on optical outputs of the signal lightsat λ5 to λ8 wavelengths (for example, when a signal light at λ8 is cutoff, signal lights at λ5 to λ7). This always makes it possible toperform a fixed and stable control of the device.

[0109] The amplified multiplexed lights and the supervisory light signalhaving passed through the optical coupler 28 are transferred to thetransmission path 7 ₁ which is a single mode transmission fiber.

[0110] A part including the rare earth-doped optical fibers 34, 63 ₃, 68₃, 38, the pumping light sources 33, 61 ₃, 71 ₃, 39, and the opticalmultiplexers 32, 62 ₃, 40 may be replaced by a semiconductor opticalamplifier. In this case, it is advisable that an amplification ratio iscontrolled by a pumping electric current instead of the pumping lightsources 33, 61 ₃, 71 ₃, 39.

[0111] In the present configuration, a signal input power into thebuffer light amplifying unit 10 ₂ or 10 ₃ from the transmission path 7 ₁or 7 ₂ falls in a range of 5 dBm to 30 dBm, and a signal amplificationgain in the buffer light amplifying unit 10 ₂ or 10 ₃ is equal to anorder of about 10 dB. Since there is furnished no optical isolatorwithin the buffer light amplifying unit 10 ₂ or 10 ₃, an attention mustbe paid to oscillation phenomena of light. Accordingly, the signalamplification gain in the buffer light amplifying unit 10 ₂ or 10 ₃should be, preferably, 30 dB or less, or more preferably, 15 dB or less.Also, by making a positive gain the signal amplification gain in thebuffer light amplifying unit 10 ₂ or 10 ₃, a noise figure for a signalinput from the transmission path 7 ₁ or 7 ₂ in the intermediate repeateris obviously improved as compared with the methods in the prior art, butmore preferably, the noise figure should be 5 dB or more.

[0112] Moreover, it is preferable that amplification gain distributionsin the core light amplifying unit 11 ₂ and the buffer light amplifyingunit 10 ₂ or 10 ₃ should be calculated from a necessary output powerinto the transmission path 7 ₁ or 7 ₂. For example, assuming that theoutput power into the transmission path 7 ₁ or 7 ₂ is equal to +11 dBmper signal wavelength, the total signal power (λ1 to λ4 and λp1, or λ5to λ8 and λp2) turns out to be +18 dBm, and consequently it ispreferable that a power of the pumping light source 33 or 39 should beset to be about 1.25 to 3.3 times as high as this power. When the powerof the pumping light source is not enough, as illustrated in FIG. 5, thefollowing units may be added, thereby providing a bi-directional pumpingfor the erbium-doped optical fibers: a new pumping light source 18-a, anoptical detector 57-a for detecting the optical output thereof, acontrol unit 58-a for keeping a detected supervisory unit signalunchanged, and an optical multiplexer 17-a for introducing the pumpinglight. Besides, in any case, it is preferable that the pumping lightsources 33, 39 which correspond to a forward pumping for the multiplexedlights from the transmission path 7 ₁, 7 ₂, are furnished.

[0113] Based on the ability of the pumping light source 33 or 39 setabove, it is possible to set an input power of the multiplexed lights,which are conveyed into the buffer light amplifying unit 10 ₂ or 10 ₃from the optical amplifier 31 or 36, at the value of the optical outputfrom the buffer light amplifying unit 10 ₂ or 10 ₃ (+11 dBm)−X dB. It ispreferable that a range of X should be 0 to 20. An adjustment of X makesit possible to set, at the above-mentioned more preferable value, asignal amplification gain for a signal input power which isreverse-directional, i.e. in a direction from the transmission path 7 ₁in the case of the buffer light amplifying unit 10 ₂, and in a directionfrom the transmission path 7 ₂ in the case of the buffer lightamplifying unit 10 ₃. Here, a 1480 nm semiconductor laser, which isadvantageous for a high power pumping, may be employed as the pumpinglight source 71 ₂ inside the optical amplifier 36, or as the pumpinglight source 71 ₃ inside the optical amplifier 31.

[0114] However, employment of the 980 nm semiconductor laser isdesirable for the pumping light source 33 inside the buffer lightamplifying unit 10 ₂, the pumping light source 39 inside the bufferlight amplifying unit 10 ₃, the pumping light source 61 ₂ inside theoptical amplifier 36, or the pumping light source 61 ₃ inside theoptical amplifier 31. As per the above description, in much the same waythe terminal station repeater 5 ₁ is applicable to the terminal stationrepeater 5 ₂, the intermediate repeater 6 ₁ is applicable to theintermediate repeaters 6 ₂, 63 ₃, etc.

[0115] With respect to a configuration of the terminal station repeaterand an intermediate repeater according to the present invention, theconfiguration blocks included therein may be located outside the bufferlight amplifying unit. For example, FIG. 12 illustrates a configurationin which monitor light multiplexers/demultiplexers 9 ₂, 9 ₃ are locatedoutside a buffer light amplifying unit. Even in such a configuration, itis possible to obtain the effects given by the buffer light amplifyingunit according to the present invention. Regarding an insertion loss ofa signal light in the monitor light multiplexers/demultiplexers 9 ₂, 9₃, it should be set to be, more preferably, 1.9 dB or less, or even morepreferably, 0.4 dB.

[0116] Illustrated further in FIG. 13, as a partial derivativeembodiment of the bi-directional optical transmission system illustratedin the above-described FIG. 11, is a configuration embodiment of abuffer light amplifying unit and core length amplifying unit in asingle-directional optical transmission system. According to the presentconfiguration, a signal light from a transmission path passes through amonitor light multiplexer/demultiplexer 9 ₂, then being introduced intothe buffer light amplifying unit. A monitor light demultiplexed by themonitor light multiplexer/demultiplexer 9 ₂ is multiplexed with aninfinitesimal an detectable-enough signal light which, being notcompletely demultiplexed, is left behind. An optical multiplexer 88extracts only the signal light from this, and a bandwidth passing lightfilter 46 and an optical detector 47 detects the signal light input. Inthe buffer light amplifying unit 10 ₂, a demultiplexed signal light,after being amplified by a rare earth-doped optical fiber 34, isintroduced into the core light amplifying unit 11 through an opticalmultiplexer 32. The rare earth-doped optical fiber 34 is the same as therare earth-doped optical fiber in FIG. 11 in that it is pumped by apumping light source 33. Also, as is the case with FIG. 11, a portion ofa signal light amplified by the core light amplifying unit 11 ispartially branched by an optical brancher 89. The signal light amplifiedby the core light amplifying unit 11 is configured to be introducedagain into the transmission path through the monitor lightmultiplexer/demultiplexer 9 ₂.

[0117] In the buffer light amplifying unit in the present configuration,there is no need of so much signal gain. This makes it possible toobtain an effect of power of the pumping light source 33 even if theoutput thereof is comparatively low. Accordingly, for example, thefollowing configuration is allowable. By regarding the pumping lightsource 33 as a pumping light source 61 ₃ or regarding the pumping lightsource 33 as a pumping light source 71 ₃, the pumping light source poweris distributed into the two light amplifying units. In that case, it isadvisable that a lower pumping light source power should be distributedinto the buffer light amplifying unit. The above-described configurationof the present invention is very effective in uni-directional opticaltransmission systems.

[0118] A simple calculation makes it possible to verify theeffectiveness of the present configuration. For example, when the signalinput is set to be −27 dBm, the value of NF according to theconventional method turns out to be 7 dB or more even if the insertionloss in the monitor light multiplexer/demultiplexer 9 ₂ is assumed to be0.4 dB and the value of NF in the rare earth-doped optical fiber isassumed to be 3.5 dB. On the other hand, the value of NF made possibleby the configuration illustrated in FIG. 13 has been found to be 3.86dB, assuming that the insertion loss in the monitor lightmultiplexer/demultiplexer 9 ₂ is 0.4 dB, a gain in the buffer lightamplifying unit 10 ₂ is 13 dB, a gain in a previous-step rareearth-doped optical fiber 63 ₃ inside the core light amplifying unit 11is 15.5 dB, a loss in the dispersion compensator 67 ₃ is 10 dB, and again in a next-step rare earth-doped optical fiber 68 ₃ inside the corelight amplifying unit 11 is 18 dB.

[0119] Accordingly, the present configuration makes it possible toreduce at least 3 dB of NF, as compared with the conventionalconfiguration. Thus, converting from the signal S/N, it becomes possibleto extend a transmission-possible distance by about 100 km or longer.Incidentally, in this trial calculation, a signal light output to atransmission fiber 7 ₂ in FIG. 13 has turned out to be +6 to 8 dBm,which is extremely close to a value in an actual system.

[0120] Another embodiment of a one way transmission equipment of thepresent invention is described using FIGS. 14 and 15. FIGS. 14 and 15are block diagrams that explain the embodiment of the one waytransmission equipment of the present invention. The difference betweenthe two different positions of the pumping light source that excites thedoped fiber of the buffer amplifier. That is, in the transmissionequipment of FIG. 14, it is backward pumping that is adverse with thetransmission direction of the signal light. On the other hand, in thetransmission equipment of FIG. 15, it is forward pumping that is thesame as the transmission direction of the signal light. Because generaland forward excitation is considered as a low noise, only FIG. 15 isexplained here. But all the contents are also common to the embodimentof FIG. 14.

[0121] The example illustrated in FIG. 15 is the example of transmissionequipment that reduced three pumping light sources 33, 61 and 71 usedwith the one way transmission equipment of FIG. 13 to two pumping lightsources and planned economization. The process that the signal light isamplified is quite similar to the embodiment of FIG. 13, and descriptionis omitted. 120 MW pumping light is conducted to coupler 120 of which abranching ration is 2:8 that excite impurity doped fiber 34 from thepumping light source 33 in the structure of this example. The pumpinglight, from the port of which the branching ratio 2 of coupler 120,excites impurity doped fiber 34 of the buffer amplifier. The pumpinglight, from the port of which the branching ratio 8 of coupler 120,excites impurity doped fiber 63 ₃ of the core amplifier. And, in thisembodiment, 0.98 μm.

[0122] The gain of the buffer amplifier is acceptable at 10-16 dB, andthe fiber length of impurity doped fiber 34 is also acceptable at 3-6 m.When the buffer amplifier is made high excitation, an optical isolatorbecomes necessary for an input step to the contrary, and it is contraryto the purposes of a present invention. And, the gain of impurity dopedfiber 63 ₃ of the core amplifier is 10-20 dB and fiber length 10-20 m.

[0123] Because in this embodiment, a fiber of which dispersion is largeis presupposed in 1.5 μm band as transmission fibers 7 ₁ and 7 ₂,dispersion compensator 67 ₃ is used. Therefore, to supply signal loss bydispersion compensator 67 ₃, the other impurity doped fiber 68 ₃ isinstalled in the core amplifier. It is clear that providing transmissionequipment for a transmission line using DSF with few dispersion in 1.5μm band, renders unnecessary the dispersion compensator 67 ₃, impuritydoped fiber 68 ₃ and pumping light source 71 ₃.

[0124] There is amplification equipment in the preceding phase ofisolator 60 ₂ in this example. As a result NF with the wholetransmission equipment can be greatly improved. NF was 7.0 dB with thedesigned transmission equipment in which a buffer amplifier was notinstalled. In comparison with this, by setting the buffer amplifier, NFgreatly improved with 4.9 dB or less. In additional, because the pumpinglight source of the buffer amplifier and the core amplifier can becommon, economical transmission equipment can be obtained.

[0125] The present invention, when applied to an optical transmissionsystem including terminal station repeaters and intermediate repeaters,makes it possible to provide an optical transmission device which iscapable of performing a long haul transmission with a high reliability.Also, the present invention makes it possible to provide a long hauloptical transmission system with high reliability.

[0126] In all embodiments described above, the relationship between thedoped fiber and the pumping light sources does not limit the structuresillustrated in the drawings. This is true even if the bi-directionalpumping, backward pumping, or forward pumping is used.

[0127] While the present invention has been described in detail andpictorially in the accompanying drawings, it is not limited to suchdetails since many changes and modification recognizable to these ofordinary skill in the art may be made to the invention without departingfrom the spirit and scope of the invention, and all such modificationsas would be obvious to one skilled in the art are intended to beincluded within the scope of the following claims.

We claim:
 1. An optical transmission device, comprising: a core lightamplifying unit; a first buffer light amplifying unit for amplifying afirst signal light from a first transmission path and an amplifiedsecond signal light from said core light amplifying unit, supplying saidcore light amplifying unit with said first signal light, and supplyingsaid first transmission path with said amplified second signal light;and a second buffer light amplifying unit for amplifying a second signallight from a second transmission path and an amplified first signallight from said core light amplifying unit, supplying said core lightamplifying unit with said second signal light, and supplying said secondtransmission path with said amplified first signal light.
 2. An opticaltransmission device according to claim 2, wherein said core lightamplifying unit comprises: a first optical multiplexer/demultiplexer; asecond optical multiplexer/demultiplexer; a first optical amplifier foramplifying said first signal light from said first opticalmultiplexer/demultiplexer so as to send out said amplified first signallight to said second optical multiplexer/demultiplexer; and a secondoptical amplifier for amplifying said second signal light from saidsecond optical multiplexer/demultiplexer so as to send out saidamplified second signal light to said first opticalmultiplexer/demultiplexer.
 3. An optical transmission device accordingto claim 2, further comprising: a third opticalmultiplexer/demultiplexer for multiplexing and demultiplexing said firstand second signal light to and from said second opticalmultiplexer/demultiplexer.
 4. An optical transmission device accordingto claim 3, wherein said core light amplifying unit further comprises:an optical coupler for coupling said second opticalmultiplexer/demultiplexer to said third opticalmultiplexing/demultiplexing.
 5. An optical transmission device accordingto claim 4, further comprising: a supervisory unit for generating asupervisory light signal which is used for monitoring the state of atransmission path; a supervisory light signal multiplexer/demultiplexer,connected between said core light amplifying unit and said first bufferlight amplifying unit, for multiplexing/demultiplexing said supervisorylight signal on a transmission path connected between said core lightamplifying unit and said first buffer light amplifying unit.
 6. Anoptical transmission device according to claim 5, wherein said corelight amplifying unit further comprises: an optical filter for receivinga light signal from said optical coupler and filtering said supervisorylight signal from said light signal; a first optical detector fordetecting said light signal from said optical filter and providing adestination signal to said first optical amplifier; and a second opticaldetector for detecting a light signal from said optical coupler andproviding a detection signal to said second optical amplifier.
 7. Anoptical transmission device according to claim 1, wherein said firstbuffer light amplifying unit comprising: an optical coupler forreceiving said first signal light from said first transmission path; apumping light source for pumping light; and an optical multiplexer formultiplexing the pumping light from said pumping light source onto atransmission path between said first buffer light amplifying unit andsaid core light amplifying unit.
 8. An optical transmission deviceaccording to claim 4, wherein said first buffer light amplifying unitcomprising: an optical coupler for receiving said first signal lightfrom said first transmission path; a pumping light source for pumpinglight; and an optical multiplexer for multiplexing the pumping lightfrom said pumping light source onto a transmission path between saidfirst buffer light amplifying unit and said core light amplifying unit.9. An optical transmission device according to claim 8, furthercomprising: a supervisory unit for outputting a supervisory light signalwhich is used for detecting a state of a transmission path; and asupervisory light signal multiplexer/demultiplexer, for multiplexing anddemultiplexing said supervisory light signal to and from a transmissionpath connected between said first buffer light amplifying unit and saidcore light amplifying unit.
 10. An optical transmission device accordingto claim 9, wherein said core light amplifying unit further comprises:an optical filter for receiving a light signal from said optical couplerand filtering said supervisory light signal from said light signal; afirst optical detector for detecting said light signal from said opticalfilter and providing a destination signal to said first opticalamplifier; and a second optical detector for detecting a light signalfrom said optical coupler and providing a detection signal to saidsecond optical amplifier.
 11. An optical transmission device accordingto claim 9, wherein said first buffer light amplifying unit furthercomprises: first and second optical filters each for filtering saidsupervisory light signal from said first light signal; first and secondoptical detectors connected respectively to said first and secondoptical filters each for detecting a light signal from the signal outputby the optical filter, wherein said first optical detector provides adetection signal to said first optical amplifier, and wherein saidsecond optical detector provides a detection signal to said secondoptical amplifier.
 12. An optical transmission device according to claim1, wherein use of said core light amplifying unit and said first andsecond buffer light amplifying units reduces noise in said transmissionpaths.
 13. An optical transmission device according to claim 10, whereinsaid noise is set to be less than 4.5 dB or less.
 14. An opticaltransmission device according to claim 6, wherein use of said core lightamplifying unit and said first and second buffer light amplifying unitsreduces noise in said transmission paths.
 15. An optical transmissiondevice according to claim 14, wherein said noise is set to be less than4.5 dB or less.
 16. An optical transmission device according to claim11, wherein said noise is set to be less than 4.5 dB or less.
 17. Anoptical transmission device according to claim 16, wherein said noise isset to be less than 4.5 dB or less.
 18. A transmission systemcomprising: a plurality of optical transmission devices each of whichcomprises: a core light amplifying unit; a first buffer light amplifyingunit for amplifying a first signal light from a first transmission pathand an amplified second signal light from said core light amplifyingunit, supplying said core light amplifying unit with said first signallight, and supplying said first transmission path with said amplifiedsecond signal light; and a second buffer light amplifying unit foramplifying a second signal light from a second transmission path and anamplified first signal light from said core light amplifying unit,supplying said core light amplifying unit with said second signal light,and supplying said second transmission path with said amplified firstsignal light.
 19. A bi-direction transmission equipment whichinterconnects a first transmission path and a second transmission path,said bi-directional transmission equipment comprising: a firstbi-directional optical amplifier which receives a first signal lightfrom the first transmission path and a second signal light from a coreoptical amplifier and provides the first signal light for the coreoptical amplifier and the second signal light for the first transmissionpath; and a second bi-directional optical amplifier which receives thefirst signal light from the core optical amplifier and the second signallight from the second transmission path and provides the first signallight for the second transmission path and the second signal light forthe core optical amplifier, wherein said core optical amplifiercomprises: a first optical amplifier which receives the first light andamplifies the first signal light, and a second optical amplifier whichreceives the second signal light and amplifies the second signal light.20. An optical signal receiver which suppresses a noise figure,comprising: a first optical amplifier which receives an attenuatedsignal light, amplifies the attenuated signal light to a firstpredetermined level and outputs the amplified attenuated signal light toan optical component, said first optical amplifier being a low gainamplifier; when said optical component attenuates the signal light andoutputs the light signal to a second optical amplifier; and wherein saidsecond amplifier amplifies the signal light at a second predeterminedlevel.
 21. An optical amplifier for amplifying an optical signal from atransmission line, said optical amplifier comprising: a first dopedfiber which amplifies the optical signal at a gain of less than 16 dB; afirst pumping light source which excites said first doped fiber; anoptical component which provides an amplified optical signal having aloss resulting from said first doped fiber; a second doped fiber whichamplifies the amplified optical signal from said optical component at again more than the gain of said first doped fiber; and a second pumpinglight source which excites said second doped fiber.
 22. An opticalamplifier for amplifying an optical signal from a transmission line,said optical amplifier comprising: a first doped fiber which amplifiesthe optical signal and has a length less than 6 m; a first pumping lightsource which excites said first doped fiber; an optical component whichprovides an amplified optical signal having a loss resulting from saidfirst doped fiber; a second doped fiber which amplifies the amplifiedoptical signal from said optical component at a gain more than the gainof said first doped fiber; and a second pumping light source whichexcites the second doped fiber.
 23. An optical amplifier for amplifyingan optical signal from a transmission line, said optical amplifiercomprising: a first doped fiber which amplifies the optical signal; anoptical component which provides an amplified optical signal having aloss resulting from said first doped fiber; a second doped fiber whichamplifies the amplified optical signal from said optical component; apumping light source which generates a pumping light; and an opticalseparator which branches the pumping light to said first and seconddoped fibers, wherein said optical separator provides a large amount ofpumping light to said second dope fiber.
 24. An optical amplifier foramplifying an optical signal from a transmission line, said opticalamplifier comprising: an optical component which provides a signal loss,wherein a noise figure of said optical amplifier is less than 6 dB. 25.The optical amplifier described in claim 24, wherein said opticalcomponent is an isolator.
 26. The optical amplifier described in claim24, wherein said optical component is a wavelength demultiplexer.